In the very recent past titanium and titanium alloys have come to play a more and more important part in technology. This is due to the outstanding technological properties of titanium materials, particularly their high resistance to corrosion and low specific gravity, which given the relatively high strength of titanium alloys, gives a weight saving of almost 40% compared with steel. Titanium and its alloys have therefore proved valuable particularly in aeronautical engineering and space travel, in chemical plant, power generation, marine technology and --owing to their good tolerance by the human body--in medical technology.
While unalloyed titanium is a ductile material with high elongation and reduction in area, its strength is increased quite considerably with increasing contents of alloying elements at the expense of ductility and formability; this applies particularly to oxygen, which brings about solution strengthening, and consequently four grades of unalloyed titanium are recognised in the art with oxygen contents of 0.05 to 0.35% and tensile strengths of 240 to 740 N/mm.sup.2. The strength is however to a large extent dependent on temperature, and falls to about 50% even at a temperature of only 300.degree. C.
Since titanium has a hexagonal crystal structure with fewer slip planes than the face-centred cubic or body-centred cubic crystal lattice, its resistance to deformation is so great that commercial .alpha.+.beta.-titanium alloys can hardly be cold-formed at all. Unalloyed titanium on the other hand is more or less cold-formable, depending on its oxygen content. However, increasing oxygen content and reduction lead to such pronounced cold-hardening that intermediate annealing becomes unavoidable. Thus for example after a 40% cold reduction the tensile strength is doubled while the elongation at fracture falls to one third. The elongation at fracture is then often only 5 to 10%. This is a great disadvantage since high surface quality and strength can only be obtained by way of cold forming, even at the expense of the ductility. Thus the unalloyed titanium with the lowest content of interstitial impurities of .ltoreq.0.10% oxygen (Werkstoff-Nr. 3.7025 according to DIN 17850) is still very easy to cold work. However, with an increasing proportion of foreign atoms, particularly oxygen, in the lattice, the cold formability is greatly reduced, so that heavy deformation is only possible with the use of repeated intermediate annealing in connection with a working cycle.
The intermediate annealing is usually performed either above the recrystallisation temperature (soft annealing at 600.degree. to 800.degree. C.) in order to restore the cold formability by forming new nuclei, or by a stress relieving heat treatment in the temperature range of from 500.degree. to 600.degree. C.
The cold forming is followed by a final heat treatment. Here the type and amount of the preceding cold work plays a decisive role. This gives rise to the possibility of obtaining a desired grain size in soft-annealing through the amount of reduction and the temperature and duration of the anneal.
According to DIN 65084 the final or soft annealing is usually performed--in dependence on the content of interstitial impurities in solution--above the recrystallisation temperature in the range of 600.degree. to 800.degree. C. and with a soaking time of 10 to 120 minutes.
If no recrystallisation is necessary, then according to DIN 65084 a stress relieving heat treatment is performed as an alternative as a final heat treatment in the temperature range 500.degree. to 600.degree. C. with a soaking time of 30 to 60 minutes.
Titanium and titanium alloys have already proved valuable in medical technology, for example as material for endoprotheses, jaw implants, bone plates, bone screws, bone needles, heart pacemaker cases and surgical instruments. Owing to its good strength properties the standard alloy TiA16V4 is outstanding. However the vanadium content of this alloy appears to cause problems, since elementary vanadium undergoes toxic reactions in the human body. While solution of the vanadium in the solid solution lattice reduces the danger of toxic reactions, this danger is not completely eliminated, particularly when friction and wear occur. Nickel-containing alloys should not be used either, since in individual cases there is then the danger of a nickel allergy. There is therefore a trend towards the use of vanadium-free titanium alloys, for example the specially developed implant alloy TiA15Fe2.5.